I know that’s not your point, I just want to point out that toddlers aren’t selfish as much as they just haven’t developed empathy yet, as a sense of empathy usually only develops after the 4th birthday. The golden rule just doesn’t work for toddlers, they can’t put themselves in someone else’s shoes and imagine how they feel. There’s a riddle/test with a doll and a closet that illustrates this well.